Accurate characterization of the brightness temperature (TB) of black-body targets used for calibrating microwave remote-sensing radiometers includes many inputs: antenna pattern and loss, target temperature, target emissivity, mechanical alignment, and radiometric TB measurements, all of which must be calibrated against physical standards. Here, we describe measurements made using several black-body targets and two different antennas within the WR-42 (18 to 26.5 GHz) waveguide band. Uncertainty estimates are also shown for the retrieved target TB measurements.

We report on the characterization of blackbody reflections as a part of the recent progress on the development of brightness standards for microwave remote sensing at National Institute of Standards and Technology (NIST). Three blackbody targets at variable temperatures used for airborne and/or satellite systems along with an aluminum plate were measured in terms of their reflection coefficients by horn antennas in connection with a vector network analyzer (VNA) in the WR-42 waveguide band. Precision measurements of reflection are needed for blackbody emissivity computation to check against the brightness temperature measurement of blackbody targets. All experiments were conducted in two distance ranges by free-space methods in an anechoic chamber. Preliminary results show negligible reflections from the calibration targets, indicating near ideal blackbody characteristics in the measured frequency range.

The Noise Project in the Electromagnetics Division of the National Institute of Standards and Technology (NIST) has proposed the development of standards for microwave brightness temperature for use in remote-sensing applications such as satellite-based weather observations. The standards would be based on existing fundamental standards for electromagnetic noise in waveguide systems. The connection to brightness temperature, which is a radiated quantity, would be made by means of a well characterized antenna. A heated calibration target would be used to supplement the basic standard, either as a check or to reduce the uncertainty by a redundant measurement. We have performed preliminary measurements at 26 GHz that demonstrate the feasibility of the proposed standard. A parallel effort is in progress for terahertz frequencies. For terahertz noise we are building a heated target to be used as a noise standard, since we did not already have fundamental noise standards for such high frequencies. This standard will be used with a terahertz radiometer. The radiometer is based on a receiver that uses a hot-electron bolometer (HEB) mixer that is coupled to the radiation by a quasi-optical adapter. We expect to perform terahertz noise measurements with the system by the end of the year.

We review the advantages of a national standard for microwave brightness temperature and outline our proposed approach toward developing such a standard. The proposal is a combined standard that would comprise both a standard radiometer, traceable to primary noise standards, and a fully characterized standard target. We also review our recent work on development of a standard radiometer and on characterization of calibration targets.

As part of the ongoing effort at NIST to develop a microwave brightness temperature standard, we are exploring the electromagnetic and thermal characteristics of microwave calibration targets. We investigate the thermal properties of a microwave calibration target using NIST infrared calibration facilities and techniques. An infrared radiometer is used to measure the radiance at the surface of a microwave calibration target and to compare it with the values reported from contact temperature sensors. We find temperature gradients within the target greater than 1 K. Infrared imagery shows temperature gradients within the geometric structure of the target.

As part of the ongoing effort at NIST to develop a microwave brightness temperature standard, we are exploring the electromagnetic and thermal characteristics of microwave calibration targets. Part of this effort is focused on measuring the electromagnetic properties of absorber materials used in the construction of microwave calibration targets. Improved materials characterization would also support applications of absorber materials beyond their use for calibration targets; for example, anechoic chamber design would benefit from this improved understanding.

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurements are described that enable one to estimate the resulting uncertainty in the measured brightness temperature. The measurement results are presented for several combinations of antenna and calibration target. The resulting uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered.

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurement results are presented that enable one to estimate the resulting uncertainty in the measured brightness temperature. This uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered

We describe the NIST effort to develop brightness-temperature standards for microwave and millimeter-wave frequencies. Results of preliminary measurements at 26 GHz are presented.